Highly doped layer for tunnel junctions in solar cells

ABSTRACT

A highly doped layer for interconnecting tunnel junctions in multijunction solar cells is presented. The highly doped layer is a delta-doped layer in one or both layers of a tunnel diode junction used to connect two or more p-on-n or n-on-p solar cells in a multijunction solar cell. A delta-doped layer is made by interrupting the epitaxial growth of one of the layers of the tunnel diode, depositing a delta dopant at a concentration substantially greater than the concentration used in growing the layer of the tunnel diode, and then continuing to epitaxially grow the remaining tunnel diode.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/404,795, filed Mar. 16, 2009.

U.S. GOVERNMENT RIGHTS

This invention was made with Government support under DE-FC36-07G017052awarded by the Department of Energy. The Government has certain rightsin this invention.

FIELD

Embodiments of the subject matter described herein relate generally to amethod for improving the electrical characteristics of aninterconnecting tunnel junction between adjacent solar cells and amultijunction solar cell having the improved interconnecting tunneljunction.

BACKGROUND

Multijunction solar cells are stacks of specifically oriented currentgenerating p-n junction diodes or subcells. When electrically connectedin series, current generated in one subcell is passed to the nextsubcell in series. Electrical characteristics of the interconnectingtunnel junction between subcells contribute to the overall efficiency ofthe multijunction solar cell.

SUMMARY

Presented is a method for improving the electrical characteristics ofthe interconnecting tunnel junction between subcells of a multijunctionsolar cell and a multijunction solar cell having the improvedinterconnecting tunnel junction. In various embodiments, the method andimproved interconnecting tunnel junction comprise a narrow, delta-dopedlayer within the interconnecting tunnel junction that improves thecurrent handling capability of the interconnecting tunnel junctionbetween subcells of the multijunction solar cell.

The features, functions, and advantages discussed can be achievedindependently in various embodiments of the present invention or may becombined in yet other embodiments further details of which can be seenwith reference to the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures depict various embodiments of the system andmethod of highly doped layer for tunnel junctions in solar cells. Abrief description of each figure is provided below. Elements with thesame reference number in each figure indicated identical or functionallysimilar elements. Additionally, the left-most digit(s) of a referencenumber indicate the drawing in which the reference number first appears.

FIG. 1 is an illustration of bandgaps of two subcells of a multijunctionsolar cell in one embodiment of the highly doped layer for tunneljunctions in solar cells system and method;

FIG. 2 is an illustration of a reversed biased junction between twosubcells of a multijunction solar cell in one embodiment of the highlydoped layer for tunnel junctions in solar cells system and method;

FIG. 3 is an illustration of an interconnecting tunnel junction betweentwo subcells of a multijunction solar cell in one embodiment of thehighly doped layer for tunnel junctions in solar cells system andmethod;

FIG. 4 is an illustration of an interconnecting tunnel junction having adelta-doped layer between two subcells of a multijunction solar cell inthe highly doped layer for tunnel junctions in solar cells system andmethod;

FIG. 5 is an illustration of an interconnecting tunnel junction having adelta-doped layer and corresponding dopant concentrations in oneembodiment of the highly doped layer for tunnel junctions in solar cellssystem and method, and

FIG. 6 is an block diagram of a manufacturing process for producing amultijunction solar cell having an interconnecting tunnel junction witha delta-doped layer in one embodiment of the highly doped layer fortunnel junctions in solar cells system and method.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature andis not intended to limit the embodiments of the invention or theapplication and uses of such embodiments. Furthermore, there is nointention to be bound by any expressed or implied theory presented inthe preceding technical field, background, brief summary or thefollowing detailed description.

Multijunction solar cells are constructed with a number of subcellsstacked one on top of the other, each subcell being a current generatingp-n junction diode. When light is incident on a subcell, photons havingenergies at or around the bandgap, Eg, are absorbed and converted intoelectrical current by the p-n junction. Photons having energies lessthan the bandgap are passed through the subcell to a lower subcell,while photons having energies higher than the bandgap are generallyconverted into excess heat. By using a topmost subcell with acomparatively high bandgap, and lower subcells of successively lowerbandgaps, more of the available spectrum from the light is available toeach subcell to be converted into electricity. Selection of thematerials in each subcell determines the available energy to lowersubcells.

Although for purposes of illustration and simplicity of explanation thefollowing figures and description describe a multijunction solar cellhaving two cells, the system and methods described herein are equallyapplicable to solar cells having one, two, three or multiple cells. Nolimitation to a two cell multijunction solar cell is implied orintended.

Referring now to FIG. 1, in one non-limiting example, a two cell solarcell 100 having a top cell 102 comprised of Gallium Indium Phosphide(GaInP) and a bottom cell 104 comprised of Gallium Indium Arsenide(GaInAs) is presented. The cells 102, 104 are grown epitaxially,starting with a Germanium (Ge) substrate, and depositing layers ofp-type GaInAs, n-type GaInAs, p-type GaInP, and n-type GaInP. Incidentlight 108 is directed at the top cell 102 of the two cell solar cell100. A portion of the light 108 is reflected 120. A portion of the light108 enters the top cell 102 and is absorbed and converted into heat 110,especially those high energy photons 112 in the light 108 having anenergy higher than the bandgap of the GaInP material, Eg>1.87 eV, of thetop cell 102. High energy photons 112 having energy approximating thebandgap of the GaInP material in top cell 102, Eg>1.87 eV, are absorbedby electrons in the GaInP material. The additional energy allowselectrons bound in the valence band of the GaInP crystalline lattice tomove into the higher energy conduction band, creating free electrons 118that contribute to the current generation of the two cell solar cell100. Low energy photons 114 have too little energy to free electrons 118in the GaInP material and pass through the top cell 102 into the bottomcell 104. Although the bandgap is illustrated as 1.87 eV, the bandgapfor GaInP may vary from approximately 1.75 eV to approximately 1.90 eV.

In the bottom cell 104, a portion of the light 108 is again absorbed andconverted into heat 110, especially those low energy photons 114 thathave an energy higher than the bandgap of the GaInAs material, Eg>1.39eV, of the bottom cell 104. Low energy photons 114 having energyapproximating the bandgap of the GaInP material in bottom cell 104,Eg>1.39 eV, are converted into free electrons 118. The remaining photons116 pass into the substrate 106 where the remaining photons 116generally are converted into heat 110. Although the bandgap isillustrated as 1.39 eV, the bandgap for GaInP may vary fromapproximately 1.35 eV to approximately 1.43 eV.

Each cell 102, 104 is comprised of a p-n junction diode that generatescurrent. The p-n junction diodes can be n-on-p type junction diodes, orp-on-n type junction diodes. Referring now to FIG. 2, two stacked cellsof n-on-p type junction diodes 200 are illustrated. Each cell 102, 104is comprised of an n-type doped layer 202 and a p-type doped layer 204.Current is generated in the n-to-p direction in each cell as illustratedby the arrow I and collected through electrical connections V+ at thetop of the two stacked cells of n-on-p type junction diodes 200, and V−at the substrate 106. By orienting the p-n junctions in both cells 102,104 in the same direction, current generated by the top cell 102 passesthrough the bottom cell, and each cell 102, 104 amplifies the voltage offree electrons 118.

However, the voltage differential between the p-type doped layer 202 ofthe top cell 102, and the n-type doped layer 204 of the bottom cellcreates a reversed biased junction 206 with a depletion region 208relatively devoid of free elections 118. As illustrated in the p-njunction voltage-current graph 210, for normal operating voltages only asmall amount of leakage current 212 is capable of flowing across thereversed biased junction 206.

Referring now to FIG. 3, to increase the current carrying capability ofthe reversed biased junction 206, an interconnecting tunnel junction, orICTJ 302, is epitaxially grown between the top cell 102 and the bottomcell 104 to create a multijunction solar cell 300. The ICTJ 302 in themultijunction solar cell 300 comprises a highly doped p+-type layer 304of GaInAs, GaInP, or AlGaAs, and a highly doped n+-type layer 306 ofGaInAs, GaInP, or AlGaAs. Through a quantum mechanical process, thehighly doped ICTJ 302 allows electrons to penetrate across the depletionregion 208, allowing an amount of tunneled current 308 to flow acrossthe reverse-biased junction 206 that is proportional to the voltage, asillustrated in the tunnel junction voltage-current graph 310.

The ICTJ 302 is constructed to pass the large amount of current thatflows between the top cell 102 and the bottom cell 104. The ICTJ 302 isoptically transparent in order to pass as much light 108 as possiblebetween the top cell 102 and the bottom cell 104. To maximizemanufacturing yields, the ICTJ 302 design is not sensitive to slightvariations common in high volume manufacturing processes. To increaseoptical transparency, the ICTJ 302 is thin and generally has a bandgap,Eg, equal to or higher than the bottom cell to avoid capturing light108, specifically low energy photons 114 (shown in FIG. 1), that wouldotherwise be converted to free electrons 118 in the bottom cell 104.However, the thinness and bandgap, Eg, requirements for the ICTJ 302limits the amount of dopants or intentional impurities (N_(A) or N_(D))that can be incorporated into the ICTJ 302. These limitations in turnlimit the peak tunneling current through an approximate relationship:

Jpeak∝exp□(−Eg3/2NA□ND/(NA+ND)))

Where J_(peak) is the product of the volume and the energy of theelectrons tunneling through the ICTJ 302 by quantum tunneling, Eg is thebandgap of the material used to grow the ICTJ 302, N_(A) is the acceptordopant concentration in the highly doped p+-type layer 304, and N_(D) isthe donor concentration in the highly doped n+-type layer 306. Note thatlower dopant concentrations and higher bandgaps reduce the peaktunneling current possible in the ICTJ 302.

In terrestrial applications, incident light 108 is concentrated andfocused on the multijunction solar cell 300. This increase inconcentrated illumination increases the current flowing through the ICTJ302. If the peak carrying capacity, J_(peak), is exceeded, a knee 312,or sudden decrease, in amount of tunneled current 308 flowing across thereverse-biased junction 206 develops, as illustrated in the tunneljunction voltage-current graph 310. In concentrator applications, theintensity of the light 108 can be the equivalent of 2000 suns or 2000times AM 1.5, a measure of spectrum and amplitude of solar radiationreaching the surface of the earth. This corresponds to a minimumJ_(peak) of 30 A/cm². The ICTJ 302 is epitaxially grown as a thin 150 Alayer (^(˜)20 atomic layers) and therefore doping levels in such thinlayers are well monitored to ensure proper dopant concentrations areachieved. However doping levels can drift during production andtherefore a design criteria for a J_(peak) Of 100 A/cm² is used toensure proper yields during the manufacturing process.

However, it is difficult to epitaxially grow a thin 150 A layer (^(˜)20atomic layers) ICTJ 302. In a typical manufacturing process, the ^(˜)20atomic layers are deposited across a 20″ substrate millions of times. A10% variability in thickness is a standard requirement. The ICTJ 302thickness therefore needs to be controlled to just + or −2 atomic layersacross the entire area of the 20″ substrate during manufacturing.Moreover, the peak amount of dopant (N_(A) or N_(D)) scales inverselywith the band gap, Eg, of the material being doped. This limits theN_(A) or N_(D) dopant concentration to the approximately 10¹⁹ cm⁻³ rangefor materials having band gaps of 1.8 to 1.9 eV and 10²⁰ cm⁻³ formaterials having a band gap of 1.4 eV.

Although using materials with lower bandgaps increases the permissibledoping levels, materials having lower band gaps have reduced opticaltransparency. Reduced optical transparency reduces both the amount oflight 108 available to the bottom cell 104 and the energy in thosephotons that are transmitted to the bottom cell 104, thereby reducingthe energy producing capability and efficiency of the multijunctionsolar cell 300.

Using materials with higher bandgaps improves the transparency, butmanufacturing the ICTJ 302 requires tight control of doping levels toachieve the minimum J_(peak) of 30 A/cm² to 100 A/cm². Direct methods ofdoping the ICTJ 302 layers is limited as the bulk doping properties ofGroup VI dopants like Te, Se, and S are limited by the presence of anatomic surface liquid layer concentration that needs to be establishedprior to doping and the overall solubility of a bulk mixture. Thesemanufacturing constraints limit the peak concentrations and limits howthin the ICTJ 302 layers can be reliably grown. Group IV dopants like C,Si, Ge, and Sn will act as both donors and acceptors negating theoverall dopant concentration, limiting their usefulness to the rangefrom approximately 10¹⁸ cm⁻³ to low 10¹⁹ cm⁻³ concentrations. Group IIdopants like Zn, Cd, and Hg tend to be mobile in the lattice, diffusingaway from high concentration regions during subsequent epitaxyprocesses. This reduces the applicability of using Group II dopants toachieve the high dopant concentrations necessary to epitaxially grow theICTJ 302.

Referring now to FIG. 4, a multijunction solar cell with a delta-dopedlayer 400 is presented. The multijunction solar cell with a delta-dopedlayer 400 comprises a top cell 102, and bottom cell 104, and aninterconnecting tunnel junction with a delta-doped layer, or δ-dopedICTJ 402. The δ-doped layer 404 is a thin, approximately 20 A in widthhighly doped layer with a peak dopant concentration of 10²⁰ cm⁻³. The δrefers to the shape of the doping profile of the δ-doped layer thatapproaches a Dirac delta function. A Dirac delta function, or δ, is afunction that is infinite at one point and zero everywhere else. Theδ-doped layer 404 is positioned in the δ-doped ICTJ 402 and adds to theeffective N_(A) or N_(D) dopants, increasing the peak tunneling currentof the δ-doped ICTJ 402 layer. The δ-doped layer 404 increases thetunnel current carrying capability of the δ-doped ICTJ 402 byapproximately a factor of two over the ICTJ 302 of FIG. 3 without aδ-doped layer 404. This is also illustrated in the δ-doped tunneljunction voltage-current graph 408, which shows the δ-doped tunnelcurrent 406 for a reverse biased δ-doped ICTJ 402 to be approximatelytwice as steep as the curve representing the tunnel current 308 for anICTJ 302 without a δ-doped layer 404.

Referring now to FIG. 5, a δ-doped ICTJ 402 is shown in an explodedview. The highly doped p+-type layer 304 and the highly doped n+-typelayer 306 adjoin to form the reversed biased junction 206. An n-typeδ-doped layer 502 is displaced within the highly doped n+-type layer 306in close proximity to the highly doped p+-type layer 304. Placing then-type δ-doped layer 502 close to the highly doped p+-type layer 304shortens the distance electrons have to travel to cross the reversebiased junction 206 and reduces the chance that electrons will recombineprior to crossing the reverse biased junction 206, improving the peaktunneling current density. A dopant concentration chart 504 illustratesapproximately the relevant dopant concentration levels for the p-dopantconcentration 506, the n-dopant concentration 508, and n-type δ-dopedconcentration 510 in the δ-doped ICTJ 402. A portion of the n-typeδ-doped concentration 510 is estimated as shown by the dotted lines asthe narrow width of the δ-doped layer makes concentrations difficult tomeasure.

In another embodiment, the n-type δ-doped layer 502 is directly adjacentto the highly doped p+-type layer 304. In other embodiments, the n-typeδ-doped layer 502 is centered in the highly doped n+-type layer 306,displaced closer to the bottom cell 104, and placed between the bottomcell 104 and the highly doped n+-type layer 306. In another embodiment,the δ-doped ICTJ 402 contains p-type δ-doped layer (not shown) in thehighly doped p+-type layer 304. In yet another embodiment, the δ-dopedICTJ 402 utilizes both a p-type δ-doped layer in the highly dopedp+-type layer 304 and an n-type δ-doped layer 502 in the highly dopedn+-type layer 306. In another embodiment, an ICTJ 302 is placed betweenthe bottom cell 104 and the substrate 106. In yet another embodiment, aδ-doped ICTJ 402 is placed between the bottom cell 104 and the substrate106. In other embodiments, one or more ICTJs 302 and/or one or moreδ-doped ICTJs 402 are placed between adjacent cells or cells and otherstructures, including but not limited to electrical connection pointsand layers, in the multijunction solar cell with a delta-doped layer400.

Although in the preceding figures and description each cell 102, 104 hasbeen illustrated as an n-on-p type junction diode, and the δ-doped ICTJ402 as a p-on-n tunnel junction, this was for illustration purposesonly. In other embodiments, there are a plurality of cells 102, 104 andeach cell 102, 104 is separated from an adjacent cell 102, 104 by aδ-doped ICTJ 402. In other embodiments, the multijunction solar cellwith a delta-doped layer 400 comprises a plurality of cells 102, 104that are p-on-n type junction diodes, and each cell 102, 104 isseparated from an adjacent cell 102, 104 by a δ-doped ICTJ 402 that isan n-on-p tunnel junction.

In various embodiments, adjacent portions of the δ-doped ICTJ 402 andthe cell 102, 104 have the same pin type doping, either both useacceptor type N_(A) dopants or both use donor type N_(D) dopants. Thenon-adjacent portion of the δ-doped ICTJ 402 has a complementary p/ntype doping. For example, if the adjacent portion of the cell 102, 104is a p-type type semiconductor material, then the adjacent portion ofthe δ-doped ICTJ 402 is also a p-type semiconductor material, and bothcomprise acceptor type N_(A) dopants. The other portion of the δ-dopedICTJ 402 is a complementary n-type semiconductor material, and comprisesa donor type N_(D) dopants. The adjacent portions of the δ-doped ICTJ402 and the cell 102, 104 use the same acceptor/donor type dopant,however in one embodiment the adjacent portions of the δ-doped ICTJ 402and the cell 102, 104 use the same dopant, and in another embodiment theadjacent portions of the δ-doped ICTJ 402 and the cell 102, 104 usedifferent dopants. In one embodiment the adjacent portions of theδ-doped ICTJ 402 and the cell 102, 104 use the same base semiconductormaterial. In one embodiment the adjacent portions of the δ-doped ICTJ402 and the cell 102, 104 use the different base semiconductormaterials. In one embodiment, both portions of the δ-doped ICTJ 402 usethe same base semiconductor material. In one embodiment, the portionsare comprised of different base semiconductor materials.

Referring now to FIG. 6, in one non-limiting example the δ-doped ICTJmanufacturing process starts by preparing 602 a Ge substrate 106. On theGe substrate 106, grow epitaxially 604 a bottom cell 104 of n-on-pGaInAs. After the bottom cell 104 is complete, begin to grow 606 thehighly doped n+-type layer 306 of GaInAs of the δ-doped ICTJ 402. Theninterrupt the grow 606 step for approximately one minute and deposit 608a flow of Si₂H₆ at 4.5×10⁻² μmol/min for a total dose of 4.5×10⁻² μmolin the vapor along with an amount of PH₃ to produce the δ-doped layer404. As an optional step, continue to grow 610 the remainder of thehighly doped n+-type layer 306. Then grow epitaxially 612 the highlydoped p+-type layer 304 of GaInAs. And finally grow epitaxially 614 thetop cell 102 of n-on-p GaInP.

In one embodiment, SiH₄ is used instead of Si₂H₆. In one embodiment, thecells 102, 104 are p-on-n cells and the δ-doped ICTJ 402 is an n-on-ptunnel junction. In one embodiment the δ-doped layer 404 is deposited608 during the step of growing epitaxially 612 the highly doped p+-typelayer 304 of GaInAs. In one embodiment, the deposit 608 step produces aδ-doped layer 404 using other methods and process steps for depositingsilicon layers as known to those of ordinary skill.

In other embodiments, each of the P-dopants is selected from one of theGroup II, IV or V elements, and each of the N-dopants is selected fromone of the Group IV or VI elements. In still further embodiments, thesubstrate, subcells, and tunnel junction materials are selected eachfrom semiconductor materials including germanium, silicon, includingcrystalline, multicrystalline, and amorphous silicon, polycrystallinethin films including copper indium diselenide (CIS), cadmium telluride(CdTe), and thin film silicon, and crystalline thin films includingGallium Indium Arsenide (GaInAs) and Gallium Indium Phosphide (GaInP).In other embodiments, the substrate, subcells, and tunnel junctionmaterials are selected from the alloys GaAs, InAs, GaP, InP, AlAs, AlP,AlGaInP, AlGaP, AlInP, GaInP, AlInAs, AlGaInAs, GaInAs, GaAsP, GaInAsP,GaAsSb, GaInAsSb, AlInSb, AlGaSb, GaInNAs, GaInNAsSb, GaInNP, GaInNAs,SiGe, Ge, ErP, ErAs, ErGaAs, ErInAs.

The embodiments of the invention shown in the drawings and describedabove are exemplary of numerous embodiments that may be made within thescope of the appended claims. It is contemplated that numerous otherconfigurations of the S-doped interconnecting tunnel junctions may becreated taking advantage of the disclosed approach. It is theapplicant's intention that the scope of the patent issuing herefrom willbe limited only by the scope of the appended claims.

What is claimed is:
 1. A multijunction solar cell, comprising: an interconnecting tunnel junction positioned between a first solar cell and a second solar cell, the interconnecting tunnel junction comprising: a first layer of a highly doped p or n type semiconductor material directly adjacent the first solar cell; a second layer of a complementary highly doped p or n type semiconductor material to that of the first layer; and a delta-doped layer having a similar p or n type doping to that of the first layer positioned between and directly adjacent to the first layer and the second layer opposite of the first solar cell and the first solar cell, or displaced within the first layer more proximate the second layer than the first solar cell, or displaced within the second layer more proximate the first layer than the second solar cell; wherein the first layer and the second layer adjoin to form a reversed biased junction; wherein the delta-doped layer is thinner than the first layer and thinner than the second layer, and has a dopant concentration higher than the highly doped, dopant concentration of the first layer and the second layer, the highly doped, dopant concentration being in a range greater than 1×10¹⁷ cm⁻³ to less than 1×10¹⁹ cm⁻³.
 2. The multijunction solar cell of claim 1, further comprising a substrate directly adjacent the first solar cell or the second solar cell.
 3. The multijunction solar cell of claim 1, wherein a dopant concentration in the delta-doped layer is approximately 10 times greater than the highly doped, dopant concentration.
 4. The multijunction solar cell of claim 1, wherein a dopant concentration of the delta-doped layer is greater than 10²⁰ cm⁻³.
 5. The multijunction solar cell of claim 1, wherein a dopant concentration of the delta-doped layer is between 10¹⁹ cm⁻³ to 10²⁰ cm⁻³.
 6. The multijunction solar cell of claim 1, wherein a width of the delta-doped layer is approximately 20 Angstroms.
 7. The multijunction solar cell of claim 1, wherein the first solar cell comprises a semiconductor material having a bandgap lower than that of the second solar cell.
 8. The multijunction solar cell of claim 1, wherein the first solar cell comprises Gallium Indium Arsenide and the second solar cell comprises Gallium Indium Phosphide.
 9. The multijunction solar cell of claim 1, wherein the interconnecting tunnel junction comprises a semiconductor material selected from the group consisting of Gallium Arsenide, Gallium Indium Arsenide, Gallium Indium Phosphide, and Aluminum Gallium Arsenide.
 10. A multijunction solar cell having an interconnection tunnel junction with a delta-doped layer prepared by a process comprising the steps of: preparing a substrate; growing epitaxially a first photovoltaic subcell on the substrate; growing epitaxially a first layer of a tunnel diode on the first photovoltaic subcell, the first layer using a dopant of an acceptor/donor type of an adjoining portion of the first photovoltaic subcell; interrupting the step of growing epitaxially the first layer of the tunnel diode; depositing a delta-doped layer of a delta dopant of the same acceptor/donor type as the dopant on the first layer of the tunnel diode, the delta dopant in the delta-doped layer reaching a delta dopant concentration substantially greater than a concentration of the dopant in the first layer of the tunnel diode; and growing epitaxially a second layer of the tunnel diode on the delta-doped layer, the second layer using a complementary dopant of a different acceptor/donor type to the dopant of the first layer of the tunnel diode.
 11. The multijunction solar cell of claim 10, further comprising growing epitaxially an additional portion of the first layer using the dopant, after depositing the delta-doped layer and before growing epitaxially the second layer.
 12. The multijunction solar cell of claim 10, wherein the first photovoltaic subcell comprises a semiconductor material having a bandgap lower than the second photovoltaic subcell.
 13. The multijunction solar cell of claim 10, wherein the tunnel diode comprises a semiconductor material having a bandgap equal to or higher than the first photovoltaic cell.
 14. The multijunction solar cell of claim 10, wherein the delta dopant concentration in the delta-doped layer is approximately 10 times greater than the concentration of the dopant in the first layer of the tunnel diode.
 15. The multijunction solar cell of claim 10, wherein the delta-doped layer has a delta dopant concentration of greater than 10²⁰ cm⁻³.
 16. The multijunction solar cell of claim 10, wherein a width of the delta-doped layer is approximately 20 Angstroms.
 17. The multijunction solar cell of claim 10, wherein the dopant is an acceptor type Group II, IV or V dopant and the complementary dopant is a donor type Group IV or VI dopant. 